The present invention relates to a method of cutting sheet material using a cutting apparatus having a cutter head and a reciprocating blade which is caused to move along a path of cut by a pre-programmed set of instructions issued to a drive system by a computer and deals more particularly with an improvement in these instructions wherein the cutter blade is caused to follow a smooth line of cut around a corner defined by a prescribed angle to effect better running of the cutter head and wherein the cutter head is caused to move long a given path of cut always at maximum velocity.
In cutting a pattern piece from a single ply or a number of plies of fabric material a computer controlled cutting machine employs a contour builder which divides a pattern shape into a series of segments designated by beginning and end points connected in succession to define a path of cut around a pattern shape. A contour is a specific collection of such segments of the pattern defined by beginning and ending points corresponding to where the forward motion of the cutting blade is stopped. These stopping points, or breaks, occur because of any number of circumstances which may be part of a cutting operation. Examples of stopping points which are included as part of the cutting operation would be points along the cutting path where the cutter blade is stopped and sharpened by appropriate sharpening means carried by the cutting blade. Another stop point would occur where the cutter blade must turn through an angle, such as a 90 degree turn, and therefore must be stopped and rotated to align the cutting edge with the new line of cut. Thus, a further stopping point would be defined by a change of direction in the path of cut requiring the cutter head where required by programming data, to lift, rotate and subsequentially plunge the cutter blade into the layup material at the newly prescribed angle.
In the past, if a contour had a given turn angle, that is one which that could be negotiated by the cutter blade without stopping and turning, or, lifting, turning and subsequentially plunging, the whole contour nevertheless would be limited by the maximum speed that the cutter blade could pass through the turn. That is, the cutter head, while following a straight line, will be able to run at a greater velocity than the velocity required to cut through a turn thereby limiting the maximum velocity of the cutter head through an entire contour to the velocity of the cutter head at its slowest point in the contour, namely, at the tight corners in the contour.
In such numerically controlled cutting machines, all motion data sent from the controller to the motors which drive the cutter head are composed of straight line movement commands, sent to appropriate drive motors as successive motion commands at uniform intervals spaced milliseconds apart from one another to move the cutting blade along a desired line of cut. These straight line movements are so incrementally small by comparison to the overall pattern path that is to be cut, that in the cutting of a curved path, the collection of these movements take on a curvature characteristic. The segments which make up a contour thus may take the form of straight lines or ones having a given curvature. However, segments of a contour most always contain corners which are made up of two straight lines which have some angle disposed therebetween. When cutting through such a corner, the previously known systems generate instantaneous changes in the directional velocities of the knife. As depicted in FIG. 1A, the line 1 illustrates a cutting path to be followed by a knife edge. The path to be followed tracks directly along the X axis until it reaches point a whereupon a Y velocity component is added to cause the cutter head to turn at the angle theta (.THETA.). At subsequent points b and c, the cornering angle gets steeper with the addition of another Y vector component to create new angle at point b of .THETA.' and another angle .THETA." at point c. As can be seen in FIGS. 1B and 1C, at the point a where the direction changes from an otherwise pure X direction path of travel, the X and Y velocity components change stepwise. That is, up to point a, the V.sub.y velocity component was zero, and at point a, the velocity in the Y direction is increased stepwise a given unit and continues to do so at points b and c coincident with the steeper angles of change. Conversely, as the angle .THETA. steepens from an otherwise horizontal path, the V.sub.x component diminishes stepwise. As shown in FIGS. 1D and 1E, X and Y acceleration components show drastic stepwise decreases or increases at points corresponding to where instantaneous changes in the V.sub.x and V.sub.y velocity components occur. The A.sub.x and A.sub.y values at points a, b, and c are perceived by the control system to be extremely large values, and accordingly the position drive motors are correspondingly driven very hard by the controller to meet this perception, thereby resulting in harsh, jerking movements being imparted to the cutter head at each point where a change in a velocity component occurs. This jerking or harsh movement has been found to cause curve overshoot, choppy circles, and overall subjects the cutter machine to mechanical shocks which over time eventually lead to the breakdown of component parts in the system.
Accordingly it is an object of the present invention to provide a method for improving the performance of a cutter head around corners whereby the cutter is moved through a turn with a smoother motion and more accurate control of the knife direction by eliminating the hitherto known problems with the mechanical shocks imparted to the cutter as a result of discontinuities in the velocity components defining the path of travel of the knife.
It is yet another object of the present invention to maximize efficiency in a cutting operation by causing cutting along a contour at maximum velocity up to a tight corner in the contour at which point, the cutting speed slows down to negotiate the corner and thereafter cutting of the remainder of the contour is effected at normal cutting speed.
It is yet a further object of the present invention to reduce mechanical wear in the cutter head by eliminating hard shocks produced by servo motors by substituting a smooth and continuous motion for repeated high force short time system inputs.